Introduction to Shell Scripting — Automating Linux with Bash (Part 1)

In this lecture series, we move into one of the most important concepts in Linux and DevOps:
the shell.

You will learn:

• What a shell is

• What Bash is

• How shell scripts work

• How to write and execute basic Bash scripts

Most importantly, you will learn how to automate tasks instead of executing commands one by one.

---

Why Shell Scripting Matters

So far, we’ve been executing Linux commands manually:

• Creating users

• Assigning users to groups

• Creating folders and files

• Changing permissions

• Installing software

• Starting applications

All of these tasks work perfectly when done once.

But what happens when:

• You need to configure 10 servers the same way?

• You want to reproduce a server setup later?

• A teammate needs the same configuration?

• You want logic, conditions, or checks before running commands?

• You want to perform bulk actions, like adding many users or renaming hundreds of files?

Running commands manually becomes slow, error-prone, and hard to track.

---

The Idea: Put Commands into a File

Instead of typing commands one by one, wouldn’t it be better if we could:

• Write them into a file

• Execute them all in the correct order

• Reuse the same setup anywhere

• Share the configuration with others

That file is called a shell script.

A shell script:

• Contains a sequence of Linux commands

• Can include logic and conditions

• Can be reused, copied, and stored

• Is typically saved with a .sh extension

---

What Is a Shell?

On Unix-like systems (Linux and macOS), the shell is a program that:

• Takes commands from the user

• Interprets them

• Translates them into instructions the kernel understands

In simple terms:

The shell is the interface between the user and the operating system kernel.

Because of this, a file that contains shell commands is called a shell script.

---

Different Shell Implementations

There isn’t just one shell. There are multiple shell programs.

Bourne Shell (sh)

• Original Unix shell

• Located at:

    /bin/sh
  

• Named after its author, Stephen Bourne

• Still available on all Unix-like systems

Bash (Bourne Again Shell)

• Improved and feature-rich version of sh

• Default shell on most modern Linux systems

• Fully backward-compatible with sh

• Supports advanced scripting features

Because Bash is now the standard, the terms:

• shell scripting

• bash scripting

are often used interchangeably.

Bash is both a command interpreter and a programming language.

---

Writing Your First Bash Script

Let’s write a simple script to configure a server.

Step 1: Create the Script File

touch setup.sh

Shell scripts typically end with .sh.

---

How Does Linux Know Which Shell to Use?

Since multiple shells may exist on a system, Linux needs to know:

• Which interpreter should run the script

This is done using a shebang line.

---

The Shebang Line

The shebang line must be the first line of the script.

For Bash:

#!/bin/bash

This line tells Linux:

Use the Bash interpreter at /bin/bash to execute this script.

You can verify the Bash binary exists:

ls /bin/bash

Or:

ls /bin | grep bash

---

Why Is It Called “Shebang”?

The name comes from the first two characters:

• # → sharp

• ! → bang

Together: shebang

---

Choosing the Shell Interpreter

• #!/bin/bash → Bash-specific features

• #!/bin/sh → Maximum portability across Unix systems

If Bash is not installed on a system and your script requires it, execution will fail.

---

Adding Commands to the Script

Open the script in an editor (for example, Vim):

vim setup.sh

Add the following content:

#!/bin/bash
echo "Setup and configure server"

Save and exit.

---

Executing a Shell Script

Try running the script:

./setup.sh

You’ll see:

permission denied

---

Why Permission Denied?

By default, newly created files do not have execute permission.

Check permissions:

ls -l setup.sh

You’ll notice there’s no x flag.

---

Adding Execute Permission

Grant execute permission to the file owner:

sudo chmod u+x setup.sh

Now:

• The file becomes executable

• The file color changes in the terminal

• The execute flag (x) appears

---

Running the Script Successfully

./setup.sh

Output:

Setup and configure server

Your shell script is now executed just like a program.

---

Alternative Way to Execute Bash Scripts

You can also explicitly use Bash:

bash setup.sh

This works even without execute permission, but the ./script.sh method is the standard Unix approach.

---

Summary

In this lecture, you learned:

• Why shell scripting is essential for automation

• What a shell is and how it works

• The difference between sh and bash

• What a shebang line is and why it matters

• How to write a basic Bash script

• How to make a script executable

• Different ways to execute shell scripts

This is the foundation of automation in Linux and DevOps.